1. Field of the Invention
The present invention relates to an image heating apparatus to be applied to image forming apparatuses of copying apparatuses, printers, etc. and more specifically to a heater to be applied to image heating apparatuses which use films.
2. Related Background Art
For convenience of description, an image forming apparatus which is used for heat fixing (fixing), as a permanent image, of an unfixed toner image formed and born on a recording material such as a transferring material, an Electrofax sheet or an electrographic recording paper by a transferring (indirect) method or a direct method will be taken as an example of image forming apparatuses such as electrophotograpgic apparatuses and electrographic recording apparatuses.
Conventionally, a heat roller type apparatus has been used frequently as an image forming apparatus. This apparatus comprises a fixing roller serving as a heat roller which is heated to a predetermined surface temperature by a built-in heat generating source such as a halogen lamp and a pressurizing roller which is in contact with the fixing roller under a pressure, leads a recording material as a member to be heated into a pressure contact nip (fixing nip portion) between the above described rollers, sandwiches and conveys the recording material in the pressure contact nip, thereby thermally fixing an unfixed toner image on a surface of the recording material with heat of the fixing roller in the pressure contact nip portion.
Since the fixing roller has a large heat capacity, loses heat in a large amount and is low in a thermal efficiency, however, the fixing roller requires a long time to be heated to a temperature suited for heating the member to be heated, is in lack of a quick start property and must be always kept at a high temperature even in a stand-by state, thereby consuming energy at a high rate contrary to energy saving. Furthermore, the fixing roller dissipates heat in the image forming apparatus even in the stand-by state, thereby posing a problem of temperature rise in the image forming apparatus.
Japanese Patent Application Laid-Open No. 63-313182 or the like therefore propose film heating type heating apparatuses as apparatuses which have quick start properties and permit energy saving as well as on-demand heating.
The film heating type heating apparatus comprises a planar heating member which has a small heat capacity (the so-called ceramic heater), a film which slides on the heating surface of the above described heating member and a pressurizing roller which forms a nip in cooperation with the heating member with the film interposed, catches a recording material in the above described nip, conveys the recording material together with the film and heats the above described recording material with heat transmitted from the heating member by way of the above described film. A heating apparatus which has such a configuration provides merit to permit configuring a heating member used as a heat source and a film for transmitting heat from the above described heating member so as to have heat capacities smaller than those of the heater and the heat roller of the heat roller type heating apparatus, thereby raising a temperature rapidly and saving electric power in a stand-by state.
FIGS. 12A, 12B and 12C are schematic configurational diagrams of the heating member: FIG. 12A being a partially cut view of a surface (to be brought into contact with a heat-resistant film 103) of the above described heating member 101, FIG. 12B being a rear view of the heating member 101 and FIG. 12C being an enlarged cross sectional view taken along a 12Cxe2x80x9412C line in FIG. 12B.
In FIG. 12A showing the surface view of the heating member, reference numeral 111 denotes a ceramic substrate which is elongated in a lateral direction and made of alumina or the like, and reference numeral 112 denotes a heating resistor which is formed like a thin belt, disposed on a surface of the substrate 111 in a longitudinal direction and made of silver palladium. Reference numeral 115 denotes electrodes which are formed on a surface of a left end of the substrate so as to be electrically conductive to a left end of the heating resistor 112 and made of silver or the like. Reference numeral 113 denotes an insulating surface protective layer made of glass or the like which covers the heating resistor 112 except locations of the above described electrodes 115 and is formed on the surface of the substrate.
In FIG. 12B showing the rear surface of the heating member, reference numerals 116 and 116 denote two thin belt like electrically conductive patterns made of silver or the like which are formed in parallel with each other on the rear surface of the substrate from a right end of to an approximately middle portion of the substrate in a longitudinal direction of the substrate, reference numeral 114 denotes a temperature detecting resistor which is formed on the rear surface of the substrate so as to establish electrical conductivity between left ends of the two electrically conductive patterns 116.
An AC voltage is applied from a power supply circuit (not shown) across the two electrodes 115 and 115, whereby the heating resistor 112 generates heat from an overall length and the heating member is rapidly heated.
A temperature of the heating member 101 is detected with a temperature detecting resistor 114 disposed on the rear surface of the substrate, an output from the above described temperature detecting resistor 114 is fed from right ends of the electrically conductive patterns 116 and 116 (DC lines) to a power supply control circuit (not shown) and power supply to the above described DC line is controlled so that the temperature of the heating member 101 is maintained at a predetermined level. That is, temperature control of the heating member 101 is performed.
Alumina is conventionally used as the ceramic of the substrate of the heating member 101 and the substrate may be broken due to a thermal stress incase of remarkably thick paper passage or double feeding where portions outside paper ends (no-paper passage portions) are temporally apart from a pressurizing roller, heat is not taken by paper and the pressurizing roller, thereby causing an abrupt temperature rise (hereinafter referred to as temperature rise of the no-paper passage portions) and producing a large temperature gradient. As a measure to prevent such breakage, it is proposed to use, as a substrate, aluminum nitride which has a heat conductivity several times as high as that of alumina so that a large temperature gradient cannot be produced. However, aluminum nitride is remarkably expensive, thereby posing a problem that aluminum nitride requires a higher cost than alumina.
It is therefore conceivable to adopt a configuration in which a substrate made of a relatively inexpensive metal is coated with glass as an insulating layer, and a heating resistor and insulating glass are disposed over the insulating layer as on the conventional heating member.
However, a metal substrate poses a problem that the substrate is relatively liable to be deformed, thereby tending to be heated uniformly in a longitudinal direction of a heating member.
When a thin metal, for example stainless steel, is coated with glass in particular and the glass is set at a high temperature at a time of calcination, another problem is posed that a heating member is remarkably warped when the heating member returns to normal temperature due to a difference in thermal expansion between the metal and glass. It is conceivable that the warping may degrade an assembling property or remarkable warping may result in breakage of a heating resistor. Simple thickening of a substrate made of a metal for enhancing its rigidity will enlarge a heat capacity of an appliance, thereby degrading a quick start property.
An object of the present invention is to provide a heater for heating an image which is not deformed easily even when a metal substrate is used as well as a manufacturing method of the heater and an image heating apparatus.
Another object of the present invention is to provide a heater which uses a metal substrate, a heater for heating an image which improves slide of a film and an image heating apparatus.
Still another object of the present invention is to provide an image heating apparatus comprising an elongated heater, and a film having a surface which slides on the heater and another surface which moves in contact with a recording material bearing an image, wherein the image on the recording material is heated by heat emitted from the heater via the film, the heater has a substrate made of a metal and the substrate has a convex portion in a longitudinal direction the heater.
Still another object of the present invention is to provide a heater for heating an image comprising an elongated substrate and a heat generating layer disposed on the substrate, wherein the substrate is made of a metal and has a convex portion in a longitudinal direction of the substrate.
Still another object of the present invention is to provide an image heating apparatus comprising a heater and a film having a surface which slides on the heater and another surface which moves in contact with a recording material bearing an image, wherein the film has an endless shape, the film is disposed inside the film, the image on the recording material is heated by heat emitted from the heater via the film, the heater includes a substrate made of a metal and a heat generating layer disposed on a surface of the substrate on a side of the film, and a surface of the substrate on a side of the heat generating layer has a curved shape which is convex outside.
Still another object of the present invention is to provide a heater for heating an image comprising an elongated substrate and a heat generating layer disposed on the substrate, wherein the substrate is made of a metal and a surface of the substrate on a side of the heat generating layer has a curved shape which is convex outside.
Still another object of the present invention is to provide a manufacturing method of a heater comprising a step of forming a convex portion on an elongated substrate made of a metal and a step of forming at least one of an electrically insulating layer, a heat generating layer and a protective layer by calcination on a substrate on which a convex portion is formed.
Further objects of the present invention will be apparent from the following description.